The mechanism of action of harmaline on renal solute transport

The effect of the hallucinogenic drug harmaline was tested on rat kidney proximal tubular solute and water transport, using in vivo micropuncture and electrophysiological techniques as well as in vitro biochemical techniques. During peritubular application harmaline (5 mmol/l) was found to block net tubular volume absorption reversibly (by 85%) through inhibition of active Na+ transport and possibly active HCO-3 transport. The inhibition was accompanied by a rapid strong depolarization of the tubular cell membranes. As a biochemical equivalent harmaline inhibited the Na+-K+-ATPase and the Mg2+-ATPase of peritubular cell membrane fractions as well as the HCO-3-stimulated ATPase of a brush border membrane fraction with similar kinetics. By studying glucose tracer efflux and by measuring cell membrane potential and conductance changes in response to glucose perfusions, no evidence for a direct effect of harmaline on Na+-glucose (or amino acid) cotransport mechanisms in the brush border could be obtained. The data suggest that harmaline does not specifically compete with Na+ for transport sites. Neither are the cotransport systems in the brush border membrane specifically inhibited, nor could the inhibition of the Na+ pump in the peritubular cell membrane simply result from a competition between harmaline and Na+.